1. Field of the Invention
This invention relates to subsea navigation, for example following or tracking of subsea structures to facilitate inspection or other operations, it further relates to surveying the position of subsea structures.
2. Description of Related Art
Performing periodic inspection and/or maintenance is essential in the operation of subsea pipeline systems. There are two main ways in which this is currently conducted, i.e. by the use autonomous underwater vehicles (AUVs) or directly by divers.
The use of AUV's is preferred but problems still arise. For example, when carrying out an inspection, the AUV must be made to follow the appropriate subsea structure closely enough so that it can provide useful information. Typically, when inspections are being carried out, the AUV will include sonar scanning equipment to form images of the structure. With such equipment there is a trade off between resolution and field of view. Therefore, if the precise position of the structure is not known relative to the AUV, the equipment must be set up with a relatively large field of view and a correspondingly low resolution to ensure an image of the structure is obtained. In most circumstances, it is either undesirable or impossible for a human operator to monitor the position of an underwater vehicle relative to the subsea structure and hence AUV's are used which attempt to follow the structure autonomously.
Existing systems use inertial navigation equipment which does not operate perfectly. Thus the further an AUV is from its start point, the further off course it may have wandered. In practice this means that the array of sonar detectors provided on the AUV has to be configured to give a field of view sufficient to cope with the maximum deviation from course which might arise during the length of the scan. As mentioned above this leads to a reduction in resolution.